Conventionally, an AC plasma display panel (hereafter called “PDP”) shown in FIG. 8 is known as one example of gas discharge panels. This figure shows a construction of a part of the PDP in perspective view, with certain parts omitted.
The PDP includes an envelope 12 that is composed of a first substrate 5 and a second substrate 10 which are opposed to each other and whose periphery is sealed with a seal member 11 consisting of a low melting glass. The first substrate 5 is formed with a plurality of display electrodes 2, a dielectric layer 3, and a protective layer 4 which are formed on the internal surface of a glass substrate 1. The second substrate is formed with a plurality of data electrodes 7 extending orthogonally to the display electrodes 2, a dielectric layer 8 which are formed on the internal surface of a glass substrate 6. In addition, a plurality of partition walls 9 consisting of a low melting glass are formed at equal spaces and in parallel on the dielectric layer 8 in order to divide an internal space between the two substrates into a light-emitting cell.
Also, a phosphor 13 is applied onto the dielectric layer 8 for each light-emitting cell divided by the partition walls 9 in order to display a color image, and a discharge gas consisting of a mixture of Ne and Xe is enclosed in the envelope 12 with approximately 66,500 Pa of pressure.
In general, the PDP is manufactured by attaching the first substrate 5 to the second substrate 10 which are fabricated by separate steps. The first substrate 5 is prepared in the following manner: that is, the display electrodes are formed on the glass substrate, and a dielectric is applied thereon as a layer and baked. Finally, a film consisting of MgO or the like as the protective layer is formed on the dielectric layer according to an electron-beam evaporation (EB evaporation) method or the like to complete the first substrate 5.
Meanwhile, the second substrate 10 is prepared in the following manner: that is, the data electrodes are formed on a glass substrate, a dielectric is applied thereon as a layer, and the partition walls consisting of a low melting glass are formed in a predetermined pattern. Next, phosphors are applied as a layer between partition walls. Finally, the seal member (normally consisting of a mixture of a flit glass and a binder) is applied onto the periphery of the glass substrate, and pre-baking is performed for driving off the binder included in the seal member to complete the second substrate.
Then, the first substrate and the second substrate fabricated as above are arranged and fixed at predetermined locations with contacting each other, while being heated and sealed to complete the envelope.
Finally, after an internal space within the envelope is evacuated, the space is heated at a predetermined temperature. Then, the discharge gas is enclosed in the space to complete the gas discharge panel.
Here, some finished PDPs which are manufactured by the above-mentioned steps have problems of increase in a discharge starting voltage, generation of an abnormal discharge phenomenon during light-emitting, and so on. These problems result from the following reasons.
Firstly, the MgO layer formed on the first substrate as the protective layer is made up of a plurality of needle shaped molecules which are arranged systematically and substantially vertical to the glass substrate. As such, if water or gaseous molecules are absorbed into these molecules, then it is difficult to remove the water or the gaseous molecules from there.
In the finished panel, the protective layer is exposed to discharge, so that the temperature of the layer becomes high. As a result, the water or the gaseous molecules gradually leak out to the discharge space, which deteriorates the degree of purity of the discharge gas.
Secondly, phosphors formed on the second substrate have an extremely porous structure. Thus, water or gaseous molecules are absorbed into the phosphors as well as the protective layer.
It can be considered that such a deterioration of the degree of purity of the discharge gas causes the above-mentioned problems of increase in the discharge starting voltage and generation of the abnormal discharge phenomenon. Naturally, it is preferable to remove both water and gaseous molecules. However, it is known that effects can be obtained when water only is removed. Therefore, it is preferable that the first substrate after forming a protective layer thereon and the second substrate after a pre-baking of the seal member are not exposed to the air as much as possible. However, it is the current state of the art that such a consideration is not given in the actual PDP manufacturing step.